Why is sickle cell anemia so prevalent in people of African descent?

In heterozygotic individuals, the gene for sickle cell anemia is protective against malaria.

Explanation:

Through the same mechanism that the mutation associated with sickle cell anemia produces the disease (altered hemoglobin structure), having a single copy of the mutation (that is, being heterozygotic) provides protection against malaria. This is believed to be due to the altered structure causing difficulty for the malaria parasite when it attempts to enter red blood cells as part of its life cycle.

Today, there are medications to successfully treat malaria, but it still killed 627,000 people in 2020. This represents a significant evolutionary pressure for genes that would prevent serious malarial illnesses. As malaria  was and still is common in African regions, the result was a higher prevalence of sickle cell anemia in those of African descent.